Circuit Breaker Sizing Calculator Formula

Understand the math behind the circuit breaker sizing calculator. Each variable explained with a worked example.

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Formulas Used

Minimum Breaker Rating

minimum_trip = min_rating

Suggested Standard Size

suggested = (min_rating <= 6) ? 6 : ((min_rating <= 10) ? 10 : ((min_rating <= 16) ? 16 : ((min_rating <= 20) ? 20 : ((min_rating <= 25) ? 25 : ((min_rating <= 32) ? 32 : ((min_rating <= 40) ? 40 : ((min_rating <= 50) ? 50 : ((min_rating <= 63) ? 63 : ((min_rating <= 80) ? 80 : ((min_rating <= 100) ? 100 : ((min_rating <= 125) ? 125 : ((min_rating <= 160) ? 160 : ((min_rating <= 200) ? 200 : ((min_rating <= 250) ? 250 : 400))))))))))))))

Variables

VariableDescriptionDefault
load_currentFull Load Current(A)40
safety_factorSafety Factor (125% typical)(%)125
min_ratingDerived value= load_current * safety_factor / 100calculated

How It Works

Circuit Breaker Sizing

The breaker must protect the cable while allowing normal load current to flow.

Sizing Rule

Minimum breaker rating = Load current x 1.25 (125% per NEC for continuous loads)

Then select the next standard breaker size at or above this value. The breaker rating must also be at or below the cable ampacity to ensure the cable is protected.

Worked Example

A 40 A continuous load with 125% safety factor.

load_current = 40safety_factor = 125
  1. 01Minimum rating = 40 x 1.25 = 50 A
  2. 02Standard size = 50 A breaker
  3. 03Verify that cable ampacity is at least 50 A.

Ready to run the numbers?

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